Biological and chemical sensor systems are of great importance for monitoring medical and environmental conditions and warfare threats. While great strides have been made in sensor technology over recent years, sensors are still too large, unreliable and expensive for widespread and easy use.
Over a decade ago nitric oxide (NO) was named molecule of the year by the editors of Science because of its many diverse roles in the environment, biochemistry and defense-related fields. Subsequent research on NO has further enhanced interest in this molecule and important chemical processes, and revealed the need for more accurate and robust NO sensors. NO and S—NO species sensors capable of long-term, continuous operation in field environments for environmental sensor applications and in vivo for biomedical applications are of great interest. Such sensors have the potential to significantly impact treatment and diagnosis of disease and the well-being of the global environment.
The concept of a derivatized surface to promote specificity to target analytes, if engineered properly, is envisioned to minimize non-specific binding events at the sensor surface and eliminate the requirement of an analyte selective membrane. While functional groups could be chosen for selective sensing (such as detection of NO), a reduction in spurious surface chemical events would limit signal noise levels and provide greater sensitivity. Of scientific interest would be verification of specificity and reduced noise levels.
GaN-based devices are anticipated to offer greater sensitivity as an active transduction platform. HFET devices are generally characterized by a well-defined two-dimensional electron gas (2DEG) layer formed in the near region of two coincident, epitaxial semiconductors. The induced polarization at this interface for III-N heterojunctions leads to charge densities as high as 1013 cm−2. An interesting consequence of the HFET design is the dependency of the 2DEG properties on the surface electronic or charge state. Because of this surface charge coupling III-V gateless HFET devices, including AlGaN/GaN, have been shown to be sensitive to the adsorption of molecules. The surface charge coupling is an intrinsic device property for III-N heterostructures, due to their polarization characteristics, while it is an extrinsic property of most other III-V HFETs. While this has led to concerns over electronic device instability, harnessing responses to adsorbed species has direct implications in sensor technology.
GaN has emerged in the last decade as an important semiconductor for a range of applications, from visible to UV emitters to microwave power amplifiers. In comparison to other more advanced semiconductor systems, such as Si and GaAs, GaN offers significant advantages for sensing, including its' robustness and relatively strong surface-coupled FET drain current. It is expected that functionalized surfaces of GaN-based sensors can be engineered to target almost any specific analyte for chemical or biological application.7 
AlGaN/GaN HEMT devices have recently shown utility in sensor applications indicating these devices to be pH-responsive (−OH and H+ sensitive) and sensitive to polar liquids, including methanol, propanol, water, and acetone. Acetone showed the greatest response which was attributed to its high dipole moment. To foster selectivity to target analytes, AlGaN/GaN HFETs have been integrated with lipid bilayer membranes as well. In comparison, passivation and chemical functionalization of AlGaAs/GaAs HFET devices have been studied to promote electrical and materials stability, with recent efforts targeting biosensor applications. Interestingly, AlGaAs/GaAs HFETs have been evaluated for sensor response to NO in order to exploit its high carrier mobility (See U.S. Pat. No. 6,433,356 and US Published Patent Application No. 2004/0072360, the entire content of each being expressly incorporated hereinto by reference). U.S. Pat. No. 6,647,796 (the entire content of which is expressly incorporated hereinto by reference) discloses an integrated microsensor which includes a bowed micromachined membrane coupled to a substrate to provide a strain-FET comprised of an AlGaN/GaN heterostructure.
Covalent attachment of iron-porphyrin (Hemin) molecules to GaAs and the corresponding functionalized sensor response have also been reported. Sensitivity to 1 μM NO in physiological aqueous solutions (pH 7.4) at room temperature was observed. In addition, the functional groups were reported to enhance GaAs material stability. Yet, concerns over the presence of arsenic for biomedical sensing applications and the existence of Fermi level pinning, which masks sensing response, have been problematic. Materials stability is a major consideration in aqueous sensing environments where surface instabilities and oxide dissolution can mitigate sensor performance.
GaN's high 2DEG conductivity, extreme corrosion resistance, and strong surface state coupling to the 2DEG (with minimal Fermi level pinning) make for an ideal FET-based sensor platform. Additionally the polarization charge on the surface of the HFET can induce accelerated functionalization and allow for a wide range of molecules to be adsorbed to the surface for selective and sensitive detection of NO. This controllable surface charge distinguishes a GaN-based sensor from one that is GaAs-based. It is our intention to functionalize the GaN surface with molecular groups that offer a host of engineering options for tailorable sensing capability. Furthermore, the materials structure of the device can be designed to operate in enhancement-mode, which will allow for more contrasting detection of NO. The device could also be designed to incorporate Saville and Saville-like detection of S—NO species. This selectivity is desirable for an NO sensor if the hemin molecule can be functionalized to the GaN surface. The addition of a semi-permeable membrane such as Nafion over the gate region could also enhance the selectivity of this device by preventing molecules such as NO2 from reaching the active surface layer. This would enable the device to be utilized in a variety of environments with the ability to “filter out” other possible analyte responses. Adsorbed NO by the hemin molecule has been shown to decrease the surface potential of GaAs, which caused an increase in current throughout the device. An optimized AlGaN/GaN device is expected have higher surface sensitivity than GaAs-based devices, allowing for more precise measurements and more reliable signals at lower concentrations of NO. AlGaN/GaN has roughly five times greater carrier density within the 2DEG compared to AlGaAs/GaAs HEMT devices. Interestingly, the AlGaAs/GaAs sensors have been shown to be reversible when exposed to a 10 ns 532 nm laser pulse. This visible wavelength could transmit through the backside of the GaN device because of its transparency, conceivably providing a means to periodically refresh the sensor.